This invention relates to the bottom-hole coring of subsurface formations and more particularly to the use of coring fluids containing bulk material for preventing invasion of whole mud solids into core samples taken from such formations.
Samples of earth material are taken from subsurface formations for various purposes. For example, samples are taken to obtain information concerning the chemical or physical characteristics of an earth material, such as permeability, porosity or fluid saturation. In the exploration for and production of oil and gas, samples of earth materials are taken ordinarily by means of a core drill and the samples obtained are in the form of cylinders or cores. Drilling muds with a water or oil base are commonly used as coring fluids. These drilling muds are normally formulated to provide desired density and rheological properties which make them particularly suitable for use in coring wells. For example, drilling muds may be altered to increase the density by adding solid materials, such as barium sulfate, thereto. During the coring of a subsurface formation, contamination of a core sample by the drilling mud can readily occur. The sample material, being porous, will be penetrated by the drilling mud filtrate under the pressure conditions present in the well. Depending on the size of the pore throats in the core material, mud solids (barite, clay minerals and rock cuttings) may also penetrate the core material.
Prior to carrying out porosity or permeability measurements in the laboratory on such a core sample, it is usual practice to clean the core sample of brine, hydrocarbons and mud filtrate. A standard cleaning method is to flush the core sample with liquid toluene and carbon dioxide gas followed by methanol extraction to remove the salt. To remove mud solids various methods have been used, such as ultrasonic cleaning, flooding and waterjet cleaning. In one method described in U.S. Pat. No. 4,687,523 to Hall et al, a dynamic evolution of a dissolved gas previously injected into a core sample in a liquid solution is utilized to carry solid contaminant particles from within the core sample to the outer surface of the core sample. These methods are not very effective in removing solids from deep inside the core sample.
It is, therefore, an object of the present invention to provide a new and improved method for coring subsurface formations in which invasion of whole mud solids into the core samples is minimized or prevented.